For the bulk of my stuff, I use photos that other wonderful folk have made available as public domain, or some other licensing that allows people to use the work as they see fit (perhaps with some restrictions).
All of my work is licensed under the Creative Commons Attribution non-commercial share-alike 3.0 License. [link]

So my work is based on someone else's work. And sometimes these wonderful people ask that you send them a link if you use their stuff. I always try to do that. (I keep expecting to get complaints - you've perverted my lovely picture!)

This lady was kind enough to post what I did with her photo on her blog. Thanks Mary, that was unexpected, and very kind.

Weird and Wonderful World of Chi: [link]

Check out the rest of her blog; what an interesting person!